Lazy holiday. Now I feel cr@p

so I've just had a fabulous week away. Didn't eat stupid amounts, but didn't log either. It was VERY hot every day so only managed two walks and half a yoga session. First day back at work today, and I feel awful. I've got a splitting headache, I couldn't sleep last night and I'm in a really bad mood.

Now some of this is just because it's my first day back, but I also think perhaps lack of exercise has got me down. Has anyone else experienced this after a week off exercising? Is the best way just to get back in the saddle even though I feel rubbish?